[PATCH iwl-net v2 5/6] ixgbe: fix ITR value overflow in adaptive interrupt throttling

[Aleksandr Loktionov <aleksandr.loktionov@intel.com>]

ixgbe_update_itr() packs a mode flag (IXGBE_ITR_ADAPTIVE_LATENCY,
bit 7) and a usecs delay (bits [6:0]) into an unsigned int, then
stores the combined value in ring_container->itr which is declared as
u8.  Values above 0xFF wrap on truncation, corrupting both the delay
and the mode flag on the next readback.

Separate the mode bits from the usecs sub-field; clamp only the usecs
portion to [0, IXGBE_ITR_ADAPTIVE_LATENCY - 1] (= 0x7F) using min_t()
so overflow cannot bleed into bit 7.

Fixes: b4ded8327fea ("ixgbe: Update adaptive ITR algorithm")
Cc: stable@vger.kernel.org
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
---
v1 -> v2:
 - Add proper [N/M] numbering so patchwork tracks it as part of the set;
   no code change.

 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index 210c7b9..9f3ae21 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -2889,8 +2889,9 @@ static void ixgbe_update_itr(struct ixgbe_q_vector *q_vector,
 	}
 
 clear_counts:
-	/* write back value */
-	ring_container->itr = itr;
+	ring_container->itr = (itr & IXGBE_ITR_ADAPTIVE_LATENCY) |
+		min_t(unsigned int, itr & ~IXGBE_ITR_ADAPTIVE_LATENCY,
+		      IXGBE_ITR_ADAPTIVE_LATENCY - 1);
 
 	/* next update should occur within next jiffy */
 	ring_container->next_update = next_update + 1;
-- 
2.52.0